1. Field of the Invention
The present invention relates to an improvement in a mask-defect inspecting apparatus for inspecting a defect, for example, braking, thinning, or adhesion of foreign matters, in a photo-mask used in the manufacture of a semi-conductor or a mask having a pattern formed on a sample of a glass wafer.
2. Description of Related Art
Conventionally, in an apparatus for inspecting a defect in a pattern formed on a photo-mask used for manufacturing a semiconductor, a chip on which a pattern is provided is observed by use of one detection optical system, and an image observed is compared with a reference image depending on design data to detect the defect of the photo-mask.
In addition, this kind of conventional apparatus for inspecting the mask defect includes a reflection-illumination optical system and a transmission-illumination optical system as the detection optical system to enable the defect in the mask to detect with higher sensitivity (for reference, see JP-A 10-97053).
Recently, in this kind of conventional apparatus for inspecting the mask defect, with a development in ministurization of semiconductor devices, a wavelength of illumination light used for exposure and inspection is significantly shortened, and deep-ultraviolet rays (DUV) have been used as the illumination light.
On the other hand, because the detection optical system used for the mask defect-inspection apparatus has a performance similar to a high-resolution microscope and uses an objective lens of a higher numerical aperture, it has higher resolving power, while has shallower depth of focus. As a result, the detection optical system has properties that the depth of focus becomes shallower as the wavelength of illumination light becomes shorter.
Therefore, this kind of conventional apparatus is designed to identify whether the detected defect corresponds mainly to the residue of the film-forming material or attachment of the foreign matter onto the mask by conducting the detection with fine adjustment of a focal plane of the detection optical system and by switching over an inspection carried out by the transmission-illumination light and an inspection carried out by the reflection-illumination light.
However, the conventional mask-defect inspecting apparatus of this type is on a tendency that the shortening of wavelength is called for and the depth of focus becomes shallower, while there is a tendency that a size in film thickness of the pattern in the mask as an object to be inspected is thicker. Accordingly, strict accuracy is demanded for the fine adjustment of the focal plane in the inspection of the transmission-illumination and the reflection-illumination, and also enlarging a range of the adjustment of the focal plane is demanded Therefore, there are troubles in prompt inspection and identification of the defect in the mask, in the conventional mask-defect inspecting apparatus.